1. Field of the Invention
This invention relates to a thick film resistive element, a thick film printed circuit board and a thick film hybrid integrated circuit device and their production methods and more particularly, to a thick film resistive element including a patterned electroconductive film and a patterned resistance film formed on an insulating substrate, a thick film printed circuit board and thick film hybrid integrated circuit device using the thick film resistive element, and their production methods.
2. Description of the Related Art
FIGS. 1a to 1e illustrate a production method of a conventional thick film hybrid integrated circuit device in the order of processing steps. The structure and production method of a conventional thick film hybrid integrated circuit device will be explained below while referring to FIGS. 1a to 1e.
First, on an insulating substrate 1 is patterned an electroconductive film 2 to form electric conductors 2a, 2b and 2c thereon as shown in FIG. 1a. As the insulating substrate 1, for example, an alumina board with a purity of 96% is used. The conductors 2a, 2b and 2c are formed in such a manner that the electroconductive film 2 is patterned on the substrate 1 by the screen printing technique using an electroconductive paste containing powdered electroconductive metal such as, for example, silver as the main component, then dried and thereafter, fired at about 850.degree. C. As the electroconductive paste to be used, such metal powder as contains silver in the alloying form as the silver-paradium alloy or silver-platinum alloy is frequently used.
Next, a resistance film is pattern-printed on the insulating substrate 1 to form a resistor 3 so as to be partially laminated on the conductors 2b and 2c at the both ends thereof as shown in FIG. 1b. That is, the resistor 3 is laminated on the conductors 2b and 2c at the both ends and contacted to the substrate 1 between such both ends. Then, respective portions 10b and 10c of the conductors 2b and 2c on which the resistor 3 is laminatedly formed constitute electrode portions. Thus, a thick film printed circuit board consisting of thus patterned electroconductive film and resistance film can be obtained. The resistor 3 is formed, similar to the case of forming the electroconductive film 2, in such a manner that using an electrically resistive paste containing a metal oxide such as, for example, ruthenium oxide as the main component, the resistance film is patterned by the screen printing technique, then dried and thereafter, fired at about 850.degree. C.
Besides, an amorphous glass film such as PbO.SiO.sub.2.B.sub.2 O.sub.3 is patterned on the substrate 1, thus forming covering members 4a and 4b for covering the conductors 2a, 2b and 2c and the resistor 3 as shown in FIG. 1c. One part 9a of the conductor 2b on the side of the conductor 2b is not covered with the covering member 4a to be exposed and on the other hand, one part 9b of the conductor 2b on the side of the conductor 2a is not covered with the covering member 4b to be exposed as well. Thus exposed parts 9a and 9b of the conductors 2a and 2b serve to act as a part-carrying pad portion for carrying assembling parts of a semiconductor integrated circuit device or the like, or as a terminal for connecting to external input/output terminals in the later steps of processing. The resistor 3 is completely covered with the covering members 4a and 4b.
The covering members 4a and 4b formed of amorphous glass serve to prevent the resistor 3 from being cracked when a laser beam is applied to the resistor 3 for trimming thereby to adjust its resistance value in the latter process. In addition, they serve to protect the surfaces of the resistor 3 and electroconductive film 2 as well as to act as a solder-resist in that a solder can be applied only to the necessary parts of the electroconductive film 2 when assembling parts are to be mounted thereon.
The covering members 4a and 4b are formed in such a manner that the amorphous glass film is patterned on the conductors 2a, 2b and 2c and the substrate 1 with an amorphous glass paste by the screen printing technique, then dried and thereafter, fired at about 500.degree. C. The firing temperature such as to be about 500.degree. C. is selected as a temperature at which the resistance of the resistor 3 being already formed is not varied. Due to the fact that the firing temperature is as low as above, the amorphous glass film thus formed is not only coarse in film quality but also porous in structure, and in some cases, small cracks may be formed therein.
Next, the resistor 3 is trimmed to adjust the resistance value, and then, as shown in FIG. 1a, an assembling part 5 is bonded through solders 6a and 6b to the respective exposed parts 9a and 9b of the conductors 2a and 2b thereby to carry it therebetween. In addition, to respective terminal connection point (not shown) is mounted an input/output terminal (not shown).
Finally, the thick film printed circuit board having the assembling part 5 thus bonded and the input/output terminal (not shown) connected thereto as shown above is dipped into an electrically insulative organic resin solution such as epoxy or phenol resin solution and heat-treated to form a protection film 7 for coating the surface of the patterned electroconductive film side of the printed circuit board. In this case, the protection film 7 covers all the conductors 2a, 2b and 2c, resistor 3, covers 4a and 4b and assembling part 5 as shown in FIG. 1e. Thus, the conventional thick film hybrid integrated circuit device is completed.
FIG. 2 is a top view partially showing the vicinity of the resistor 3 of the device shown in FIG. 1 before the assembling part 5 is mounted onto the exposed parts 9b and 9c of the conductors 2b and 2c.
With the thick film hybrid integrated circuit device as shown above, if the protection film 7 is not coated thereon, suitable moisture resistance cannot be provided, so that there arises such a problem that the insulation resistance between the conductors will be gradually degraded while being operated under high humidity environmental condition. This can be considered to be attributed partially to the structure of the films covering the conductors and the nature of the amorphous glass film.
In the above-explained thick film hybrid integrated circuit device, when considered with the protection film 7 removed, as clear from FIGS. 1e and 2, the electrode portions 10b and 10c of the conductors 2b and 2c where the resistor 3 is laminatedly formed thereon are covered with the resistor 3 itself and the covering member 4b. The part-carrying pad portions 9a and 9b of the conductors 2a and 2b are covered with the solders 6a and 6b, respectively. Besides, though not shown, the terminal connection point of each conductor is also covered with solder. The other portion of the conductor 2a than the part-carrying pad portion 9a is covered with the covering member 4a only. The other portion of the conductor 2b than the part-carrying pad portion 9b and the electrode portion 10b is covered with the covering member 4b only.
As explained above, when considered with the protection film 7 removed, the conventional thick film hybrid integrated circuit device is that the electrode portion of each conductor is covered with the resistor and the covering member of members, the part-carrying pad portion and terminal connection point of each conductor is covered with the solder film only, and the other portion of each conductor than the electrode portion, part-carrying pad portion and terminal connection point is covered with the amorphous glass film only. This amorphous glass film is, as shown above, porous and yet, may be hair-cracked in some cases, which means that if the protection film is not provided, it may be easily degraded by entering the moisture into the porous amorphous glass film while being placed under high humidity environmental condition.
The electrode portion of each conductor is covered with the resistor highly dense in film quality, and the part-carrying pad portion and terminal connection point thereof are also covered with the high density solder, which means that if the moisture is entered thereinto, it is positively prevented by the resistor or solder from being further entered, thus being capable of preventing it from being reached to the conductor. In this case, however, in the other portion of each conductor than the part-carrying pad portion and terminal connection point, the moisture may be further entered through the porous amorphous glass film and/or the boundary between the glass film and substrate, finally going to the conductor itself. As a result, the metal atoms in the conductor are caused ionized by the moisture entered, and the metal ions thus obtained are attracted to the electric field applied between each adjacent conductors, and gradually move from one conductor through the porous amorphous glass film toward the other conductor as time passes, finally reaching to the other conductor. This is a process that the insulation resistance between the conductors is degraded.
When observed an insulation resistance degraded resistor which is formed between adjacent conductors microscopically, such a trace that the metal ion moves can be found on the amorphous glass film, which shows that the above considerations are true.
The explanations were made above on the degradation of insulation resistance between the conductors caused by ionization of the metal contained in the conductor with the resistor as an example, but such phenomenon may be occurred even at any portion other than the electrode portion having the resistor bonded if the electric field is applied between the conductors.
As a result, with the conventional thick film hybrid integrated circuit device as shown above, there arises such a problem that the protection film 7 cannot be omitted because of the fact that a satisfactory reliability on the moisture resistance of insulation characteristic cannot be obtained only using the amorphous glass covering film.
In addition, with the production method of a conventional thick film hybrid integrated circuit device, the protection film 7 is coated by dipping process into an organic resin solution, so that a large quantity of such solution must be unavoidably used, resulting in an difficulty to reduce the production cost. In addition, if any defective point is found out in the assembling part 5 after coating the protection film 7, it cannot be disadvantageously replaced with, which is another reason that the production cost cannot be reduced.
This invention was made to solve the above-mentioned problems, and an object of this invention is to provide a thick film resistance element and thick film printed circuit board which are superior in moisture resistance characteristic to conventional ones and do not need to coat an organic resin to form a protection film, and their production methods.
Another object of this invention is to provide a thick film hybrid integrated circuit device which does not need to coat a protection film of an organic resin and which even if any defect is found out in an assembling part after assembled, it can be easily replaced with a new one, and its production method.